Radio frequency (RF) communication systems and applications, such as wireless voice and data applications, require power amplifiers that are both efficient and linear. However, both characteristics often work against each other, such that highly linear power amplifiers are often very inefficient, whereas more efficient amplifier designs sacrifice linearity. Conventional power amplifiers generally operate at maximum efficiency at or near saturation, which affects their linearity.
Therefore, in order to accommodate RF communication signals, that have varying amplitudes, the power amplifiers within a wireless communications system are often operated in different modes, depending upon the amplitude of the input signal that is to be amplified. For example, power amplifiers might normally operate below peak efficiency for a substantial portion of their time. To address such amplification needs, Doherty amplifiers, and other amplifiers that have enhanced efficiency, such as Enhanced Class AB (ECAB) amplifiers, are utilized. For example, U.S. Pat. Nos. 6,922,102 and 7,064,606 illustrate amplifiers, which provide a desirable balance of efficiency and linearity.
However, such ECAB/Doherty amplifiers also have distortion characteristics that illustrate a phase variation, based on frequency. These distortion characteristics might be considered a spinning of the AM/PM characteristic, where the AM/PM characteristic rotates counter-clockwise with frequency. For example, the AM/PM curves for an amplifier might be reviewed using a network analyzer, which is swept over power within the desired frequency range. In reviewing the AM/PM curves over several frequencies, the inventors determined that the delay of the amplifier reduces as input power increases. This kind of distortion is most difficult to correct over wide bandwidths, as it generates an effect that is often assumed to be a memory effect. Therefore, while ECAB/Doherty amplifiers provide a desirable tradeoff between efficiency and linearity, they also exhibit distortion characteristics that are undesirable.